Shirish, The answer is yes you can use plywood if you want, the small speakers are not going to create much vibration.
Your system has three components. two speakers that have handle the mid range and high frequencies and the sub woofer that produces the low frequencies. The speakers, (the smaller ones) that typically sit on the desk can be placed on a shelf. The shelf can be made out of plywood or solid wood or particle board, it does not matter to much because your speakers (tweeters and mids ) are not very big and the higher frequencies do not vibrate very much.
The important thing is the the bracket be attached to the wall securely and the shelf material also be attached to the brackets securely. Some speakers have holes on the back so you can hang them on the wall with a mounting system or a even a screw.
Your sub woofer is going be the thing that vibrates things in your house. Low frequency sound will travel though walls and floors. You do not need it on your desk, if fact it may vibrate your desk and the things on it to much. it can be anywhere on the floor. They make special feet for speakers and subs to raise it off of the floor or shelf, these are mainly to optimize sound quality. Test it out and if it vibrating your floor move it around to find a better place. Again the low sounds travel very well so it can be anywhere.